T-square



p 1970 G. s. DQLGORUKOV 3,526,039

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m KEW United States Patent Office 3,526,039 Patented Sept. 1, 1970 3,526,039 T-SQUARE Gregory S. Dolgorukov, Ferndale, Mich. (407 Fisher Building, Detroit, Mich. 48202) Application Oct. 13, 1966, Ser. No. 586,568, now Patent No. 3,398,449, dated Aug. 27, 1968, which is a continuation of application Ser. No. 288,577, May 15, 1963, now abandoned, which is a continuation-in-part of application Ser. No. 178,868, Mar. 12, 1962, and Ser. No. 855,826, Nov. 27, 1959, now abandoned. Divided and this application June 3, 1968, Ser. No.

Int. 01.15431 7/00 US. Cl. 33112 4 Claims ABSTRACT OF THE DISCLOSURE A T-square having a one-piece arm of transparent acrylic plastic whose surfaces are optically fiat and all of whose surface intersections have been chamfered for approximately .020 inch to remove material subject to initiation therein of minute cracks which would grow into the acrylic material and cause nicking and breakage of the arm.

The present application is a division of my copending application, Ser. No. 586,568, filed Oct. 13, 1966, now Pat. No. 3,398,449 granted Aug. 27, 1968, which is a continuation of my then copending application Ser. No. 288,577, filed May 15, 1963, now abandoned, which is a continuation-in-part of my applications Ser. No. 178,- 868 filed Mar. 12, 1962, now Pat. No. 3,103,071 granted Sept. 10, 1963, and Ser. No. 855,826, filed Nov. 27, 1959, now abandoned.

The invention relates to geometric instruments and more particularly to an improved method of making and repairing a T-square adapted for use, particularly but not exclusively, with a drawing board.

An object of the invention is to provide an improved method of making and repairing a T-square having transparent guiding edges made of acrylic resin material and smoothened by removing some of the material, particularly with the use of rotatable cutters or by sanding, means being provided to eliminate the inherent susceptibility of such edges to crumbling out or nicking as well as to chipping along their sharp line edges, even in the normal use of the T-square, and breakage of their corners.

A further object of the present invention is to provide an improved method of making and repairing a T-square having its arm made of cast sheet of transparent acrylic resin having optically flat surfaces, providing an arm' of nndistortable transparency, and having guiding edges smoothened by cutting or sanding, with such edges having greater resistance to crumbling out and to nicking than conventional method of making T-squares having their entire arms, or only their guiding edges, made of acrylic resin and smoothened 'by cutting.

A still further object of the present invention is to provide an improved method of making and repairing a T- square having a head and an arm, said head and arm being secured together by a joint which eliminated any possibility of loosening of the arm or the head, which loosening frequently occurs in conventional T-squares and creates a very annoying condition, making the T- square virtually unuseable.

A still further object of the present invention is to provide an improved method of making and repairing a T-square having a head and an arm connected together by a joint which is strong and durable beyond the possibility of failure under conditions of normal use, including conditions of use that may be expected in schools.

A still further object of the present invention is to provide an improved method of making and repairing a T-square having at least its edges made of acrylic resin forming corners at the free end of the arm, which corners, nevertheless, have high resistance to breaking OE and t0 chipping off.

A still further object of the present invention is to provide an improved method of making and repairing a T- square in which the use of wood is completely eliminated, thereby eliminating the disadvantages, such as those due to absorption or drying out of moisture, different coefiicients of expansion of wood and plastics, separation of plastic from wood at the cemented joints, wearing out of paint or varnish finishes, and the like.

A still further object of the present invention is to provide an improved method of making and repairing a T-square having a head of such configuration as to produce maximum practicable contact area between the contacting surfaces of the arm and of the head and thus to ensure the strongest possible joint produced at said area by the method disclosed herein.

A still further object of the present invention is to provide an improved method of making and repairing a T- square made of acrylic plastic, which T-square does not include any extruded or molded plastic parts distorting the image underneath and causing eye strain.

A still further object of the present invention is to provide an improved method of making and repairing a T-square fabricated by cutting operations from solid sheets or plates of acrylic resin, and having an improved construction whereby the crumbling out of sharp edges is overcome and largely eliminated in a simple and effective manner, thus producing a stronger and more durable guiding edge.

A still further object of the present invention is to provide an improved method of making and repairing a T- square of the character specified in the preceding paragraph, in which guiding edges including sharp line edges causing undercutting and breakage of graphic pencil points are eliminated.

It is an added object of the present invention to provide an improved method of making and repairing a T- square of the foregoing character which has only two parts permanently secured together, and which thus is of the simplest possible construction, which is pleasant to handle, convenient to use, and is relatively inexpensive to manufacture.

A still further object of the present invention is to provide an improved method of making and repairing a T-square of the foregoing nature in which the tendency of plastic T-squares to 'breakage of the blade adjacent the T-square head is virtually eliminated, thus producing in view of other numerous advantages of my improved T-square an exceedingly durable instrument capable of withstanding even the roughest usage to which they may be normally subjected, such as that in schools.

A still further object of the present invention is to provide an improved method of making and repairing a T-square having a head and a solid plastic arm or blade secured thereto in any suitable manner, such as with the aid of screws or with the aid of an adhesive joint, in which T-square the transition from the rigid head to the flexible arm is made more gradual by stiffening and/ or reinforcing the blade at the juncture thereof with the head and thus eliminating the weak spot possessed by such T- square at the place of juncture.

A still further object of the present invention is to provide an improved method of making and repairing a T- square having a head and a solid acrylic plastic arm secured thereto, in which T-square excessive flexibility or whip" in the arm if eliminated in the locality where such whip. does not present any functional advantage butimposes stresses and strains on the arm which may prove to be beyond its strength.

A still further object of the present invention is to provide an improved method of making and repairing a T- square comprising a head and a solid acrylic plastic arm secured thereto, improved means being adapted to resist downward bending of the blade by being put in tension and carrying a substantial portion of the load produced by the force causing such bending, as well as to resist upward bending of the arm by said means operating to stiffen the blade immediately adjacent the T-square head.

A still further object of the present invention is to provide an improved method of making and repairing a T- square having a head and a solid acrylic transparent arm or blade, which T-square retains to a certain degree the accustomed appearance of the T-square made of wood and having plastic lined guiding edges, and which, nevertheless, alfords all of the advantages of a fully transparent blade and does not have any of the disadvantages of a wooden blade such as covering a material part of the drawing and separating it into two parts, as is the case with conventional opaque wooden arms.

A still further object of the present invention is to provide an improved method of making and repairing a T -square made of solid acrylic plastic and including a small piece of wood in the form of a veneer overlay which piece is so selected as to its size and is so disposed within the structure of the T-square as to eliminate the disadvantages caused in conventional T-squares by wood, such as stresses and strains imposed on plastic becausev of the difference between the coefficients of expansion of the respective materials, with the cumulative effect of such difference in the entire length of the arm causing cracking of the plastic.

A still further object of the invention is to provide an improved T-square having a head and an arm overlying the head in which the disadvantages of conventional T- squares, such as annoying or pain-producing pressure on the users left hand is completely eliminated by an expedient enhancing ornamental appearance of the T-square.

A still further object of the invention is to provide an improved method of making and repairing a T-square including an arm, the free end of which has corners made of plastic material, which corners are shaped to eliminate the chipping and produceable in a simplified manner.

A still further object of the present invention is to provide an improved T-square affording opportunities of imparting to the T-square a striking ornamental appearance.

A still further object of the invention is to provide an improved method of making and repairing a T-square having a head made of wood, plastic or any other suitable material and an arm or blade made of solid acrylic or similar plastic material and secured to the head with the aid of screws or similar connectors, improved means being provided to eliminate the tendency of the arm of such a T-square to break at the head through the holes provided in the arm for the passage of such connectors.

Further objects and advantages of this invention will be apparent from the following. description and appended claims, reference being had to the accompanying drawings forming a part of this specification, wherein like reference characters designate corresponding parts in the several views.

FIG. 1 is a top view showing an improved T-square embodying the present invention.

FIG. 2 is an elevational view of the T-square of FIG. 1.

FIG. 3 is an enlargement of the portion of FIG. 2 encircled within the circle 3.

FIG. 3a is a fragmentary sectional view taken in the direction of the arrows on the section plane passing through the section line 3a-3a of FIG. 2.

FIG. 4 is a perspective view showing the free end of the arm of the T-square of FIGS. 1-3.

FIG. 5 is a perspective view illustrating the condition of the acrylic plastic material, such as used for the arm of my improved T-squares, after such material has been subjected to the action of a milling cutter for smoothen ing of its edges.

FIG. 6 is a top view of the piece of material illustrated in FIG. 5.

FIG. 7 is a fragmentary end view taken in the direction of the arrow 7 on the piece of the material illustrated in FIG. 5.

FIG. 8 is a view similar in part to FIG. 7 but showing an edge having a chamfer of a modified construction.

FIG. 9 is an exploded view showing in section an edge of a still further modified construction.

FIG. 10 is a top view similar in part to FIG. 1 and showing a T-square of a modified construction embodying the present invention.

FIG. 11 is a sectional view taken in the direction of the arrows on the section plane passed through the section line 11-11 of FIG. 10.

FIG. 12 is a fragmentary top view showing on an enlarged scale the portion of the fixed end of the T-square arm encircled in FIG. 10 with a circle identified by the numeral C FIG. 13 is a side view of the portion of the construction shown in FIG. 12.

FIG. 14 is a View showing on an enlarged scale in FIG. 10 by a circle C the free end of the arm of the T-square of FIG. 10.

FIG. 15 is a side view of the portion of the construction shown in FIG. 14.

FIG. 16 is a fragmentary sectional view taken in the direction of the arrows on the section plane passed through the section line 16-16 of FIG. 14.

' FIG. 17 illustrates a modified form of overlay, namely, an elongated strip of fabric material adapted to be heat bonded to the plastic blade of the T-square.

FIG. 18 is a view similar in part to FIG. 10 but showing a T-square including a plastic arm secured to the head with the aid of screws and embodying the present invention.

FIG. 19 is a view similar in part to FIG. 18 but showing a still further modification thereof.

It is to be understood that the invention is not limited in its application to the details of construction and arrangement of parts illustrated in the accompanying drawings, since the invention is capable of other embodiments and of being practiced or carried out in various ways within the scope of the claims. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and not of limitation.

While the present invention is illustrated and described with reference to a T-square having an arm made of acrylic plastic, it will be understood that the invention is applicable to other plastic materials having similar physical properties, and particularly flexibility, hardness, and susceptibility to cracking as possessed by acrylic plastics. Accordingly, the term acrylic plastic as used in the specification and claims is to be understood as referring to plastic materials of such nature.

Particularly, it shall be understood that in describing the working or guiding edges of T-squares, which edges in my improved T-square include both the guiding surfaces of such edges as well as their surfaces produced by chamfers, I refer to such edges as guiding edges. On the other hand, the sharp edges, which are produced by two surfaces meeting at an angle and producing such sharp edge, 1 term line edges. Thus, the term line edge as used herein designates a geometric line at which such surfaces meet. Thus, an unchamfered guiding edge of a conventional T-square has two line edges. On the other hand, the ehamfered guiding edge of my improved T- square has four or more line edges.

In accordance with the present invention I eliminate the use of wood as a material for the construction of T- squares. I provide an improved T-square made of solid cast sheets or plates of acrylic resin plastic, with the head and the arm of the T-square being secured together by a non-loosening joint which is strong and durable beyond the possibility of failure in normal use. The construction of the T-square, particularly of the arm thereof, includes edges from which the material at the sharp or line edges thereof having the inevitable minute cracks caused by the milling cutter is removed, producing not only an edge free of such minute cracks showing a tendency to grow but also an edge in which the included angle of its surface is much larger than 90 being approximately 135. The free end of the arm is constructed in such a manner that breakage of its corners and chipping off of the end edges, inevitable in conventional T-squares having acrylic plastic edges or entire arms, is reduced to a minimum, producing a T-square uncomparably more durable than conventional T-squares.

Referring specifically to the drawings, the T-square illustrated therein comprises generally a head H and an arm A secured together as at to provide a unitary construction. The head H is made by profiling from a cast plate of acrylic plastic, preferably of solid black color. The configuration of the head, while resembling that of a conventional T-square head, is more rounded, particularly in places indicated by the numeral 11. Therefore, the head H conforms much better to the configuration of the palm of the user and thus ensures a more convenient hold on the T-square in use. Furthermore, the configuration of the curved edge of the head H is such as to increase the width of the head in the middle thereof, as compared with that of conventional T-squares, and thus to increase the area of the contacting surfaces of the head and of the arm for the purposes explained below. The ends of the head are rounded as indicated at 13. with the tangents to the end curves, at their junctures with the straight guiding edge 14 0f the T-square, forming angles a with said edge 14, with the vertices of said angles a being directed toward the arm of the T-square. I prefer to make these angles greater than 90 in order to protect the ends of the head from breaking off or chipping. T-squares having said angles a equal to approximately 110 give very satisfactory results and have a pleasant appearance.

A bevel, such as indicated by the numeral 12, made to a depth of approximately .060, eliminates the sharp corner along the curved edge of the head H at the upper surface thereof, and also contributes to the advantage of a more convenient hold on the head. A bevel similar to the bevel 12 may also be provided along the curved edge of the head H at the lower surface thereof, if desired. The bevel 15, which may be of the same magnitude as the bevel 12, is provided also along the top of the straight edge 14 for the purposes explained further below.

I prefer to make the head H somewhat thinner than wooden heads of corresponding size. For T-squares having arms less than 15 in length, I prefer to use heads 6" long and 7 to thick. T-squares having arms 15" to 28 give good results with heads 7 /2" long and A" thick. Heads 9" long and A and even thick are sufficiently thick for T-squares having arms between 28" and 30" in length. For T-squares with arms 30" and 36 long, heads 12" long and thick give very good results.

The arm A is made of a cast sheet of acrylic resin. Such sheets are made by casting them between two plates of optically flat glass or other material and, therefore, they have corresponding optically fiat surfaces, producing no distortion of the image underneath. On the other hand, molded and extruded plastics do not have such optically flat surfaces ensuring undistorted transparency, and their surfaces, being somewhat wavy and streaky, distort the image underneath and cause eye strain in prolonged use. Arms of /3" in thickness give excellent results in lengths from 18" to 36" and 2" to 2 /2" in width. Arms of less than 18" may be made %g" or .10 thick and l /21%" in width.

The guiding edges 20 and 21 of the arm A are smoothened with the aid of rotatable cutters in suitable milling machines to produce smooth and straight edges. The end edges 22 and 23 of the arm A are similarly smoothened, and their corners are rounded on a suitable radius, such as as indicated at 22a and 23a.

In accordance with the invention, in addition to such rounding of the corners, the end 23 is also beveled, preferably at a 45 angle, at both its top and bottom surfaces to the thickness of approximately .040 as is best shown in FIG. 4 at 23b. It is of importance that the bevels 23b extend not only along the edge 23 but also around the corners 23a, and extend, in effect, for a short distance into the longitudinal edges 20 and 21. I have found that merely rounding the corners is wholly insuflicient. Corners which are merely rounded even on such a large radius as 4 chip very badly, and after a perior of use look about the same as the broken off square corners. This fact has been accepted in the art as a necessary evil unavoidable in use of acrylic plastics. I have found that such chipping results from the fact that in a downward fall a T-square does not fall with its arm strictl erpendicular to the floor, and, therefore, the free end of the arm hits the floor with the arm being somewhat inclined. Therefore, the free end of the arm almost always hits the floor with the sharp edge of the rounded corner. Chipping of the corners under such conditions takes out rather large pieces of plastic, and it is just as. objectionable as breaking off of square corners. The bevel 22b may be identical with the bevel 23b, but it is provided only at the upper surface of the T-square and for different purposes. An important purpose served by said bevel 22b is to eliminate sharp edges at that end and to produce a more comfortable hand hold on the T-square head. a

It is of great importance for the purposes of the present invention that the sharp line edges or corners of the guiding edges 20 and 21 are chamfered for a depth of approximately .020" or more, both at the top and at the bottom of said guiding edges, thus producing a guiding edge shown in FIG. 3a wherein said chamfers are indicated by the numeral 25. Provision of such chamfers, in accordance with the invention, has several exceedingly important advantages. In the first place the angles b formed between the surfaces forming line edges 20a and 20b are increased by virtue of the provision of such chamfers from to producing a guiding edge uncomparably stronger and better resistant to nicking from mechanical causes, such as hitting the edge in use on some object. Furthermore, each guiding edge in my improved T-square is, in effect, dulled. Therefore, it does not undercut the pencil point and does not cause its breakage in drawing long lines as is common with cut acrylic plastic edges of conventional T-squares and other drafting instruments, such as triangles. This particular advantage is disclosed in greater detail in my said copending applications. Chamfering the sharp square line edges of the arm also makes handling of the T-square much more pleasant. It is well known in the art that hand and finger cuts may be caused by sharp edges of cut acrylic plastic edges.

Of particular importance is the function of the provision of such chamber in removing the material along the line edges of the T-square arm. For the reasons explained before I discarded the conventional understanding that sharp line edges are not objectionable since, presumably, they not only convey more clearly the impression of precision in a new instrument, but lose their sharpness after several months of use. I have found that in cutting plastics of increased hardness and brittleness, such as acrylic plastics, any milling cutter, no matter how sharp, produces separation of the material by, in effect, tearing a portion of it off from the general mass. Such tearing off of the material produces concentrated stresses particularly at the sharp edges of the sheet and origination therein of minute cracks which may extend only. as much as .003" or .004" inwardly of the plastic sheet. However, such cracks have a tendency to grow and thus to extend further and further into the mass of the material. Should two such cracks occur relatively close to each other and at a large angle, such as 90, to each other, as indicated at 26 and 27 in FIGS. and 6, they soon meet, whereupon the material between them crumbs out producing a nick. Such nicks are often mistaken for those produced by hitting and, therefore, avoidable by careful use of the instrument. I have found that origination of such nicks is wholly independent of the conditions of use, and that such nicks may occur even in new instruments, particularly those that have been stored for some time.

By providing the chamfer which may be approximately .020"-.030" in depth, I remove the material including the minute cracks, thus eliminating the above-described conditions. It should be appreciated at this point that cutting the chamfer, such as 25, does not produce similar cracks along the lines 20a and 2012. First, the force extend by the chamfering cutter on the plastic in the process of chamfering is extremely light as compared to a force exerted by the cutter cutting through the entire thickness of the arm strip. Therefore, exertion of such force on the plastic mass does not produce distortion of said mass sufficient to rupture the material. Furthermore, the angles 12 being greatly increased as compared to the angle 0 do not cause such high concentration of stresses in the line edges, and whatever concentration may occur thereat is distributed between the two stronger edges and in the mass of material between them. It should be understood that bevels 22b and 231) have a similar effect and advantages, in addition to those disclosed above.

The chamfered edges may also be buffed, thereby producing a glossy surface at said chamfered edges and strengthening them by producing a surface possessing a property known as surface tension. Although all of the chamfers described above may be provided at 45 angles to the meeting surfaces, it shall be understood that chamfers of other angles may also be used with success. Two chamfers producing a meeting line edge may also be utilized. For example, two chamfers as illustrated in FIG. 8 may be provided at 30 angles e to the surfaces meeting along the line edge 29, whereby all three internal angles 7 are increased to 150, producing a still stronger construction. Furthermore, a chamfering cutter producing rounding of the corners rather than providing a straight chamfer may also be used with success. It shall be also understood that if provision of the chamfer is desired both at the top and the bottom surface of the head or the arm, particularly at the ends of the arm and along the curved edge of the head, the same may be produced with the aid of a special cutter illustrated in FIG. 9 and designated thereby by the numeral 35, and producing an edge having two rounded corners. Such an edge is illustrated in FIG. 9 and is designated therein by the numeral 36. Similar guiding edges may also be provided on the arm.

In accordance with the invention as illustrated in FIGS. 1-9, for the purposes of securing the arm to the head I discard the use of separate connectors, such as screws, bolts, and the like, although such connectors have been used not only for wooden heads and arms but also for connecting plastic arms to wooden or plastic heads. Use of such connectors, unless combined with the features of construction hereinafter disclosed, has a number of serious disadvantages in all of such applications. I have also found that the place of juncture or contact area between the head and the arm of conventional T-squares may not provide suflicient area to produce a fully reliable joint by other methods. Because of the increased width of my improved head at the middle thereof, such area is sufliciently increased and, therefore, a cemented joint provided therein is not only much stronger than that produced with the use of mechanical connectors, but is also much stronger than it could be otherwise, irrespective of the method.

My improved joint is fully nonloosening. This is a very important advantage, since loosening of the joint bet-ween the head and the arm may cause the user to operate for considerable length of time, prior to the T-square becoming unuseable, with a T-square in which the arm may move slightly and unnoticeably with respect to the head and produce a mistake in a drawing which may be rather serious, but 'which, for a considerable length of time, could be attributed to some other cause.

I have also found that the opinion of those skilled in the art to the effect that such a joint is not practical in a T-square and should not be attempted is based on a misconception. Such a misconception may be due to the fact that in cases of molded parts the slight waviness of their mating surfaces causes such a large number of places in the joint where air is entrapped or where no contact occurs in cementing as to produce an unreliable joint. I have also found that this difliculty increases with increasing the area of the joint and may make such a joint prohibitive even in a relatively small area such as presented in a conventional T-square, marking further increasing such area objectionable. However, by using an increased area produced by the contact of two optically flat surfaces I produce a superior and unexpectedly strong joint.

I prefer to produce such a joint by using a solvent effective to dissolve and soften the surface of acrylic resin plastic and to produce, in effect, intermingling of the molecules of the two parts. Such an expedient produces a joint approaching in strength that of the virgin mass of the material. Actual experiments with my improved T-square have shown that loosening of such a joint is an impossibility, and that in attempts to separate the parts, either the head or the arm breaks first.

The bevel 15 provided along the straight guiding edge of the head H, and extending in the assembled condition of the T-square both under the arm A and on both sides thereof, performs in my improved T-square three im portant functions, as described below.

First, it clears any nicks that may be found along the line edge of the guiding edge of a drawing board, as mentioned. Such nic-ks or raised spots may be produced by separation of splinters of wood at said edge in wooden drawing boards, and by protrusion of the metal at the nicks of metal guiding edges provided in some T-squares.

The second important function of this chamfer is to eliminate the objectionable effect of the small bubbles or tears of plastic produced by the solvent squeezed out from between the contacting plastic surfaces in producing the joint. In some cases the solvent, which has already dissolved some of the plastic material, comes out at the guiding edge of the head under the arm in the form of a rounded tear drop. Such a ball or drop may retain some of its shape and upon solidification of the joint remain as a raised spot under the arm, protruding beyond the guiding surface of the guiding edge. In addition, such a tear drop, which upondissolving some of the plastic material acquires the properties of a cement, retains in itself a piece of foreign matter which freezes in place. Such a piece or crumb of foreign material forms a protrusion causing rocking of the head on the edge of the drawing board. While such rocking may be hardly noticeable in the head, it is multiplied manifold in the arm, causing rocking of its free end for a considerable distance and thus producing an unreliable T-square. By virtue of the provision of bevel 15, such protrusions are housed within the space of the recess and do not adversely affect prevision of the T-square. For this particular pur pose of the invention, the bevel should extend at least under the arm and for only a short distance on both sides thereof. It may also be in the form of a square or corner recess. However, I prefer to provide such bevel at a 45 angle and along the entire length of the guiding edge of the head H in order to increase the included angles of its two line edges to 135 and thus greatly improve the strength of the guiding edge. This is the third advantage, and it is of importance.

T-squares of solid acrylic plastic constructed as disclosed above in spite of their much lighter construction proved to be greatly superior to wooden T-squares with acrylic plastic lined edges, both with respect to durability, reliability and their convenience in use. Contrary to expectations of those skilled in the art, they withstand rough school use with negligible breakage. By tracing virtually each and every actual case of breakage within the region of predominant use of applicants equipment, applicant has found that Wherever the breakage of this T-square occurred, it was generally due to some extraneous circumstances such as intentional abuse, repeated dropping on cement floors, particularly early in the moming after the T-square had become particularly cold in an unheated room during a cold winter night. By carefully tabulating the cases of such breakage, applicant found that it occurred most frequently in the arm immediately adjacent the head.

The improvements disclosed below are intended to reduce to a minimum the possibility of such breakage first by decreasing the tendency of the arm to bend and thus cause high localized stresses and strains in its outer fibers and particularly in the sharp corners thereof. My application, Ser. No. 178,868, now Pat. No. 3,103,071 issued on Nov. 10, 1963, disclosed an improvement decreasing occurrence of localized stresses when such bending occurs due to various causes, and has proved to be a very valuable expedient. The present application discloses a further improvement whereby the tendency of the arm to bend or occurrence of loads resulting in localization of stresses is reduced, thus reducing the possibility of occurrence of such localized stresses. Accordingly, the improvements disclosed below can be used in combination with the expedients disclosed in my said copending application or independently thereof.

FIG. 10 illustrates a T-square constructed in accordance with the invention. The T-square illustrated therein comprises a head H and an arm or blade A which, with the exception of additional features desecribed below in detail, may be the same as those in the T-square illustrated in FIGS. 1-9. The joint between the head H and the arm A of the T-square may also be of the same nature to provide a bond through the entire area of contact between the head and the blade, with or without rounding the corners of such bonded area as disclosed in my said Pat. No. 3,103,071.

In accordance with the invention, at the top surface of the arm A at the fixed end thereof there is provided an overlay generally indicated by the numeral 40 and extending longitudinally of the arm from its beveled edge 41 over the guiding edge 42 of the head and for a substantial distance toward the free end of the arm. The exact length of the overlay 40 depends on the size of the T-square. Generally, it is desirable to provide an overlay extending beyond the guiding edge 42 for approximately the width of the head or somewhat further in order to provide a substantial bonded area between the piece 40 and the arm A. The exact dimensions may be modified for the purpose of giving the overlay piece 40 and, therefore, the entire T-square a pleasing appearance. The width of the piece 40 is smaller than the width of the arm A in order to leave transparent edge portions 43 similar in appearance in that portion of the arm to those of the wooden arms with plastic lined edges. The right hand end of the T-square may be made tapering as indicated at 44. With a T-square having an arm 24" long and 2%" wide, an overlay piece 1%" by 5 /2" proved to be very satisfactory.

It is of importance that the overlay piece 40 can resist a substantial load in tension. It is desirable, therefore, that the material of the overlay piece have greater resistance in tension than the plastic material of the arm, i.e., has a greater coefficient of elasticity than the material of the arm in order for the piece to assume a substantial portion of the load imposed on the arm by a force tending to bend the arm downwardly. Such overlay may be a piece of strong fabric, a piece of wood veneer, or of another plastic. If wood veneer is used, it is important that the wood grain of the veneer runs longitudinally of the arm to add a further advantage of being resistant to bending of the arm upwardly. The thickness of the wood veneer should be selected in view of its resistance to tension along the grain. I found that wood veneer approximately .040" thick is very satisfactory.

It is also of importance that connection of the overlay piece 40 with the arm A be substantially throughout the entire area of the contact and be as uniform throughout such area as practicable. Various glues or cements may be used for such purpose. A solvent suitable for the plastic material of the arm may also be used, providing that it does not cause crazing of the plastic. I found that various rubber base cements commonly known as contact glues, give good results. Where a fabric piece such as the one designated by the numeral 45 in FIG. 17 is used, the same may be treated on its underside with a suitable glue and heat bonded to the blade. The use of ready made strips of such nature intended for repairing clothing and commonly known as iron-on patches may also be used.

I prefer the use of wood veneer with the grain running longitudinally of the arm since such veneer affords strong resistance in tension with little stretching and is also sufficiently rigid in the thickness mentioned above to resist upward bending of the arm. It should be appreciated that downward bending of the arm is resisted by putting in tension the overlay piece, be it of 'Wood or fabric, and by putting the bonded area thereof in shear, thus disturbing the tension on the overlay piece throughout the entire area thereof. On the other hand, the upward bending of the arm would tend to put the overlay piece in bending and compression. It is obvious that a fabric piece cannot resist bending and, therefore, such pieces are less advantageous in this particular respect.

Furthermore, 'wood veneer can be finished in an attractive manner to show wood grain and imparts to the T- square, at least in the vicinity of its head, the accustomed appearance of a wooden T-square having a plastic head and transparent plastic edges but leaving the working portion of the blade fully transparent, thus eliminating the disadvantage of opaqueness of wooden T-squares. Walnut, mahogany, birch, and many other types of veneer are now staple items in the trade and are relatively inexpensive. An overlay of wood veneer stiffens the arm where its bending tends to be the greates and resists bending of the arm both in the upward direction as well as in the downward direction. It also makes transition from the rigid part of the T-square to the flexible part thereof more gradual. Tapering of the overlay piece at its right hand end makes such transition in the arm itself also more gradual and, therefore, is advantageous from the standpoint of strength.

Considerable problems have been experienced in the past because of the difficulty of holding conventional T- squares at their heads. The corners of the fixed end of the T-square arm pressing on the hand of the user during prolonged use of the T-square cause painful spots "requiring holding the T-square head only with the fingers rather than with the entire hand, all in the belief that such an unnatural and inconvenient hold is the solution for such problem. Such belief was caused by the fact that attempts to bevel the end edge of the arm, no matter how heavy, or rounding the corners thereof not only did not prove effective but aggrevated the problem. FIGS. 12 and 13 illustrate an improved feature of construction completely eliminating such a difliculty in an exceedingly simple and effective manner. In accordance with the invention the end edge of the arm is first beveled, as indicated at 41 in FIG. 10, whereupon the longitudinal sides of the arm at such corners are beveled with a 30 beveling cutter at an angle between 20 and 30 to the sides of the arm to produce triangular areas 47, as indicated in FIGS. l0, l2 and 13. With such a con- 'struction the user can handle the T-square for long periods of time without noticing any inconvenience whatsoever.

FIGS. 10, and 14-1 6 illustrate a modified construction of the free end of the arm intended to attain the advantages of the construction of FIG. 4. While the construction of FIG. 4 proved exceedingly effective, its production involves a number of complications and may require several operations. The construction shown in FIGS. 10, and 14-16 produces a free end of the arm of three-dimensional curvature to attain the advantages of the construction of FIG. 4, with such configuration susceptible of being produced in a single operation.

As can be seen in FIG. 14, the free end of the arm A has configuration including a substantially straight portion 50 and angular portions 51 merging with said edge 50 at well rounded portions 52. As can be seen in FIGS. 15 and 16, such curvilinear configuration is of a rounded cross section throughout its entire length, which may be attained with the use of a cutter such as shown in FIG. 9. A particular advantage of this construction is that it can be produced in one operation, with the arm being guided by a template, and produce proper blending in spite of the fact that due to manufacturing variations, arms may vary as much as /8 and even more in their width. With the use of configurations including blending-in arcs, the difference in width of the arms due to manufacturing variations makes proper blending of the corners with the straight portions of the arm very difficult and produces defects such as dips, and the like. The construction of FIG. 15 eliminated such a disadvantage.

It should be appreciated that the fixed end of the arm may also be given a similar configuration. However, instead of a rounded cross section, its cross section should be substantially square at the bottom with heavy bevel at the top.

FIG. 17 illustrates an overlay piece made of substantially unstretchable fabric and having its underside treated with a suitable glue for hot bonding the piece to the T-square to be disposed thereon in a manner similar to the piece 40 of FIG. 10. Such fabric piece can be cut off from rolls of fabric tape commonly sold for the purpose of clothes repairing. Since such fabric pieces do not impart stiffness to the arm, tapering the end thereof as shown at 44 in FIG. is less important, and merely cutting off the sharp corners thereof as illustrated at 48 may be suflicient.

FIGS. 18 and 19 illustrate an improved T-square in which a solid or one-piece acrylic plastic arm 55 is connected with the aid of threaded connections, such as screws 57, to the head 56, which head may be of plastic, wood, or other suitable material. As mentioned above, with conventional constructions breakage of such arms through the screw holes made such constructions virtually impractical. In accordance with the invention, this difficulty strength, such as fabric, wood veneer, and the like, bonded through its entire area to the head. In FIGS. 18 and 19 such overlay pieces are designated by the numerals 58 and 59, respectively. The piece 58 extends over the joint between the arm 55 and the head 56 through a distance approximately equal to the width of the head. In other words, the overlay piece 58 extends over the fixed end of the arm 55 in its portion overlying the head 56. The width of the piece 58 may be substantially the same as that of the arm 55. It will now be seen in view of the foregoing that tightening of the screws 57 would cause the screw heads to bear not against the surface of the plastic directly producing the danger of cracking the plastic, but over the surface of the overlay pieoe cushioning and distributing the pressure more uniformly. The fixed end of the arm 55 is also greatly reinforced by the provision of the piece 58 bonded thereto. The piece 59 is similar in its shape to piece 40 of the construction of FIG. 10 and is also bonded to the surface of the arm A throughout substantially the entire area of such piece, with the wood grain running longitudinally of the arm.

It will be understood that with some cements bonding of the overlay piece to the plastic may not produce a continuous or unbroken bond area but a multiplicity of small bonding spots distributed through the area of contact. While the occurrence of dry spots between such bonded spots does increase the strength of the joint, it, nevertheless, affords attainment in a substantial degree of the advantages disclosed above. However, full advantage of such expedient may be better realized by improving the bond between such surfaces to produce a substantially unbroken bonded area without interruption or dry spots.

It will also be understood that the constructions disclosed herein are exceedingly advantageous for repairing solid acrylic T-squares and particularly T-squares such as illustrated in FIGS. 1-9 should the same break adjacent their heads. To repair such a T-square, the surfaces of the arm at the break are first covered with a suitable cement, glue or solvent and are brought together, with the guiding edges as well as the top and bottom surfaces of the arm being properly aligned. After the joint is dry, such repair may be sufficient by itself. Some broken T- squares have been so repaired and have been used with success for a long time. However, bonding an overlay piece such as 40 or 45 over the break in the manner explained above will greatly reinforce the T-square at the break joint and will bring the strength of the repaired T-square virtually to that of a new T-square. T-squares having one-piece acrylic plastic arms secured to a head with the aid of screws should they break at the arm through the screw holes may be similarly repaired to produce the constructions shown in FIGS. 18 and 19.

In view of the above-disclosed construction, the objects of the present invention listed above and numerous additional advantages are attained.

I claim:

1. A T-square comprising a head having a straight and squared guiding surface and a one-piece arm secured to said head and having improved resistance to nicking and to breakage of its arm in use especially at the T- square head,- the one-piece arm of said T-square comprising an elongated strip of cast and annealed acrylic plastic having optically flat top and botttom surfaces and straight milled sides providing point-guiding and triangle locating surfaces, the four sharp line edges formed at the intersections of said surfaces being chamfered for approximately .020" in order to increase the angle of surface intersections at said edges and to remove the material thereof subject to initiation therein of minute cracks having tendency to grow into the acrylicmaterial and to cause nicking and breakage, the free end of said arm having convex cross section and having its sharp 13 vertical corners and the sharp line edges at said corners and at the end at both top and bottom surfaces of the arm removed.

2. The T-square defined in claim 1 and including a strip of Wood veneer laminated to the top surface of the arm to extend over the portion of said arm and further toward the free end of the arm, said veneer strip having width smaller than that of the arm and the length approximately two and a half times the width of the head and its grain running longitudinally of the arm.

3. The T-square defined in claim 2 the width of the veneer strip tapering at its end pointing toward the free end of the arm.

4. The T-square defined in claim 2, with the head of the T-square being milled of cast acrylic plastic sheeting and having all of its sharp line edges produced by milling relieved, said head and the arm being permanently laminated together.

References Cited UNITED STATES PATENTS 470,754 3/ 1892 Hartig. 1,082,626 12/1913 Foerst 33l07 1,458,152 6/1923 Schenck 33-112 2,265,695 12/1941 Lee 33107 2,366,449 1/1945 Keuffell et a1. 33112 FOREIGN PATENTS 896,418 1953 Germany.

OTHER REFERENCES Product Engineering, January 1949, pp. 172, 33-112.

SAMUEL S. MATTHEWS, Primary Examiner 

